Transistor bias circuit



Dec. 10, 1963 HOOTEN, JR 3,114,111

TRANSISTOR BIAS CIRCUIT Filed Sept. 12, 1958 DE TECTOR LIFIE INVENTOR BASIL P. HOOTEN, JR.

BY bwm rymu A770 EYS Uite States This invention relates to a circuit for biasing a transistor and more particularly to such a circuit for biasing a transistor which is included as a part of a radio receiver or other circuit employing vacuum tubes. 1

Transistors are often employed in conjunction with vacuum tubes in circuits such as receivers. For example, a transistor may be employed as the audio power output stage of a receiver for use in automobiles, in which a plurality of vacuum tubes are employed in a conventional manner to make up the remainder of the receiver. In such a circuit a biasing resistor is normally employed between the emitter of the transistor and the power supply. The resistor is of a low value and its purpose is to improve thermal stability at the expense of power gain. Other resistors are employed to provide a forward bias for the transistor and these resistors usually must have temperature coetficients selected to reduce the forward bias at elevated temperatures in order to achieve transistor stability. Since high values of the first mentioned biasing resistor reduce the power gain it is desirable to keep this resistance low. However, when the resistance is low it is necessary to use high temperature coefficients in the resistors employed to produce the forward bias. This increases the cost of these components.

In such a circuit as referred to above there is another defect associated with the use of a transistor as a power amplifier. This is the production of an audible pop in the loudspeaker when the receiver is turned on, due to the instantaneous conduction of the transistor which causes a step voltage to be applied to the loudspeaker.

It is an object of this invention to provide a means for increasing the reduction, at elevated temperatures, of the forward bias of a transistor used in conjunction with vacuum tubes, without increasing the resistance of the biasing resistor in the emitter circuit.

It is a further object of the invention to eliminate or reduce the sharp sound which occurs in the use of such a circuit when the circuit is activated due to the instantaneous conductivity of the transistor.

It is another object of the invention to provide the above benefits without increasing the complexity or expense of the circuit.

These and other objects and advantages of the invention are realized in a circuit in which the filament current for the vacuum tubes of the circuit is supplied through the resistor in the emitter circuit of the transistor.

In the drawing, the single FIGURE is a schematic diagram partly in block form showing an embodiment of the invention.

Referring more particularly to the drawing, there is shown a receiver of a conventional type used for broadcast reception in automobiles. The receiver comprises an antenna 1, an RF. amplifier 2, a converter 3, an IF. amplifier 4, and a detector and driver 5. Each of these portions of the receiver, following the antenna, employs a separate vacuum tube. A transistor 6 of the PNP variety is also shown as constituting the power output stage of the receiver.

The output of the driver circuit is applied by way of a transformer 7 to the base of the transistor. The collector is connected to the upper terminal of auto transformer 8, the lower terminal of which is grounded. The voice coil ate t of a loudspeaker 9 is connected across the low portion of the transformer coil 3.

The base electrode of the transistor 6 is connected to one terminal of the secondary to the transformer 7, the other terminal of which is connected to ground by way of a resistor 10. The emitter electrode is connected by way of resistor 11 to the positive terminal 12 of a source of supply voltage which in the ease of an automobile receiver would be in the order of 14 volts. The terminal 12 is also connected to a tap on a resistor 13 connected to the junction of the resistor Ill and the secondary of transformer 7.

In accordance with the invention the emitter electrode is also connected by way of a conductor 14- to a parallel arrangement 15 of the filaments of the vacuum tubes of the portions .2, 3, 4 and 5 of the illustrated circuit. The remaining terminal of the circuit 15 is grounded.

In the operation of the above circuit the resistors 10 and 13 provide forward bias between the base and emitter electrodes of the transistor. The resistor 13 is normally provided with a negative temperature coefficient or the resistor 1'1 is provided with a positive temperature coefficient of sufficient value to provide transistor stability at elevated temperatures. The resistor 11 assists in this task by providing negative feedback. This action is, of course, at the expense of power gain.

Since high values for the resistor 11 reduce the power gain, it is desirable to keep this resistance low. However, with low resistance it is necessary to use higher temperature coefficients in the elements It or 13. This increases the cost of the circuit.

In accordance with the invention the filament string for the vacuum tube portions of the receiver is connected across the resistor 11. The addition of the filament string places the resistor 11 in a power supply return circuit having a lower resistance than the emitter-collector resistance of the transistor. The current flow through the resistor and thus the voltage drop across it are accordingly increased. Typical values in a circuit such as the one illusrtrated, without the modification according to the invention, are as follows:

Resistor 11:50

Voltage drop across 11:.25 v.

Emitter current=.5 a.

.55 V. 25 C. .55 %:.44 v. 55 C.

.3 v. 25 C. .19 v. (ID 55 C.

After modification according to the invention:

Resistor 11:5

Voltage drop across ll1=.75 v.

Voltage drop across 13:}

Base to emitter voltage=-{ Emitter and heater current=1.5 a.

1.05 v. 25 C. l.05 80%=.840 v. 55 C.

Voltage drop across 13={ Base to emitter voltage Patented Dec. 10, 1963 its steady state value the transistor begins conducting in a gradual manner so that no pop is heard.

While the illustrated embodiment of the invention showsits use in a receiver all stages but one of which utilizes a vacuum tube, its use is not restricted to such a receiver. It may be used in receivers in which several stages are transistorized, or even where all stages are transistorized, if the filament of a pilot lamp is available to be connected as shown. It may also be used in other than receiver circuits, where a tr-ansistorized stage would be benefitted by similar treatment.

What is claimed is:

In a hybrid circuit including a filament heated vacuum tube stage, a transistor stage connected in common emitter circuit configuration for amplifying a signal from said vacuum tube stage and a bipolar direct current source, transistor biasing means for rendering said transistor stage inoperative during warm-up of said vacuum tube, comprising -a resistor connected between the transistor emitter electrode and one pole of said source, a voltage divider connected across both poles of said source and having an intermediate tap connected to the base electrode of said transistor thereby estab- 4 lishing the bias across the transistor base-emitter junction as the difference between the potential at said tap and the voltage drop across said resistor,

a load connecting the collector electrode of said transistor to the other pole of said source for producing an output signal,

means for injecting a signal from said vacuum tube into the base-emitter circuit of said transistor, and

means connecting the vacuum tube filament-between the transistor emitter electrode and the other pole of said source whereby increased current through said filament during warm-up increases the voltage drop across said resistor and reduces the base-emitter bias of said transistor "to prevent operation of said transistor for the duration of said increased filament current.

References Qited in the file of this patent UNlTED STATES PATENTS 2,810,071 Race Oct. 15, 1957 2,866,859 Stanley Dec. 30, 1958 2,946,014 Byles July 19, 1960 2,970,213 Dukat Jan. 31, 1961 

